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Old 12-10-2011, 08:27   #1
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Catamaran Structural Resistance to Materials Fatigue

Dear fellow Cat owners,

Pls yr attention to the following: Last week i was sailing of the coast of Brazil between Paranagua and Santos, when in the early evening the wind (SE picked up reaching 28 knots sustained with 32 knots gusts) and so did the swell (from various directions). I swiftly put in the second reef on the main and adjusted the genoa as well. (i own a 40ft FP). With swell increasing, the bouncing and pounding increased as well (plus the ocasional pounding at the wetbridge..) Inside the cabine(s) it became a pretty noisy situation (pounding) and the noise (cracking/moving??) of some wooden panels inside the cabines... I maintained a rather high course in order not to increase the boat speed (at 6 knots aprox)... I then wondered: how much pounding can a Cat actualy take? And this cracking/moving noise? Does say "fatigue" of the material come into play after a while? I have all confidence in my boat (built in 2004) but still would like to learn if somebody has some more insights on this question.
In advance, thank you very much.
Best rgds, stay safe and enjoy...
Pieter Kommerij
SV Onda Boa
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Old 12-10-2011, 11:42   #2
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Re: Catamaran Structural Resistance to Materials Fatigue

I recall a thread on this forum of osmosis discovered on the first 100 Lavezzi 40s built and recall a couple of structural delamination issues. This might account for the cracking noise.

I sincerely hope your boat does not have either issue.
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Old 12-10-2011, 12:44   #3
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Re: Catamaran Structural Resistance to Materials Fatigue

Two hulls apart 8-10 meter from each other, a huge mast in between, terrible bangings under the bridge in high seas and strong winds, to be honnest is scaring. I sailed quite a bit in various cats under these conditions and sometimes pushing too much. There have been times when I thought the boat would split in two.And yet, this has never happened to any cat anywhere even to those that are believed to be lightly built. Maybe the forces acting are not as serious as we think, who knows..

Tropic cat, not only Lavezzis but almost all of the recently built FP's cats have got osmose issue. But I couldn't quite get the relation between osmosis and cracking noise..

Cheers

Yeloya
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Old 12-10-2011, 14:20   #4
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Re: Catamaran Structural Resistance to Materials Fatigue

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Originally Posted by Pieter View Post
... And this cracking/moving noise? Does say "fatigue" of the material come into play after a while?...
Yes, fatigue occurs when a material is subjected to repeated loading and unloading (cyclic stresses). If the loads are above a certain threshold (though considerably below the normal strength), microscopic cracks (progressive brittle cracking) will begin to form at the surface. Eventually a crack will reach a critical size, and the structure will suddenly fracture.

Although the fracture is of a brittle type, it may take some time to propagate, depending on both the intensity and frequency of the stress cycles.

Nevertheless, there is very little, if any, warning before failure if the crack is not noticed. The number of cycles required to cause fatigue failure, at a particular peak stress, is generally quite large, but it decreases as the stress is increased.

The shape of the structure will significantly affect the fatigue life; square holes or sharp corners will lead to elevated local stresses, where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.

Of course, none of the above proves that the distressing sounds you describe indicate/prove critical fatigue stress.
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Old 12-10-2011, 15:03   #5
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Re: Catamaran Structural Resistance to Materials Fatigue

I was under the impression that a lot if not all of the creaking are the bulkheads and other interior wood structures moving. Most of the interior structures in those boats are in slots in the grid, free to move a small amount.
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Old 12-10-2011, 16:06   #6
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Re: Catamaran Structural Resistance to Materials Fatigue

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I was under the impression that a lot if not all of the creaking are the bulkheads and other interior wood structures moving. Most of the interior structures in those boats are in slots in the grid, free to move a small amount.
And how do the interior structures move, unless the exterior structures are also moving?
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Old 12-10-2011, 16:56   #7
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Re: Catamaran Structural Resistance to Materials Fatigue

I believe it was Chris White that wrote. Steel flexed 10 million times loses 80% of it's strength. While cold molded only lose 40%. I cannot remember what the stats for glass were.

Think of it this way, the inside of a boat is the interior of a drum. I have had horrible pounding in a storms on my mono. We wondered if she was coming apart. The cat pounds more often, but usually heading up, or off a few degrees will quiet it down. Sometimes even stop it. It's one of the compromises. Some cats pound horribly while others hardly ever do........i2f
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Old 12-10-2011, 17:05   #8
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Re: Catamaran Structural Resistance to Materials Fatigue

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Originally Posted by Sailmonkey View Post
Most of the interior structures in those boats are in slots in the grid, free to move a small amount.
Really? Which ones?
I have only ever seen them coved and glassed in
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Old 12-10-2011, 17:59   #9
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Re: Catamaran Structural Resistance to Materials Fatigue

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Most of the interior structures in those boats are in slots in the grid, free to move a small amount.
Quote:
Originally Posted by cat man do View Post
Really? Which ones?
I have only ever seen them coved and glassed in
For two, my C&C 29, & Mirage 26; and many/most of the production boats on which I've worked.
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Old 12-10-2011, 18:09   #10
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Re: Catamaran Structural Resistance to Materials Fatigue

During our circumnavigation, I listened to the wood creaking on the interior of our Privilege 39 catamaran. We have done our share of pounding and have sailed in heavy seas with winds to fifty knots three times when sailing offshore. We have never discovered any structural problems in our Privilege 39. We are a robust catamaran, and I feel confident in winds to fifty knots so far.
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Old 12-10-2011, 18:11   #11
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Re: Catamaran Structural Resistance to Materials Fatigue

Cycle fatigue effects everything. Too much flexing too many times causes failure. I would bet your noises are moving interior structure.... as a result of bending of the boat. Mono's bend too, I would imagine cats twist diagonally quite a bit. Thats why I like cats with a very big radius where the bridgedeck fairs into the hull. It's stiffer and distributes the stress over more area. It is a good question: How many cycles is FRP good for?
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Old 16-10-2011, 17:44   #12
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Re: Catamaran Structural Resistance to Materials Fatigue

Hi Pieter,

If you look at your Lavezzi you will see that many of the interior panels are covering the gaps that are built into the hull by design.
When the boat moves around these panels slide over structure and can make a creaking noise.
I believe this is part of a clever design which allows movement of the structure to prevent stress concentration points.
As far as osmosis and structural issues on FPs I have done a lot of searching on this issue and the only thing I have found is gel coat blistering.
This quite common on FPs but it is not structural, though I have no doubt some will claim it is or will lead to structural deterioration.
Interestingly I have been told osmosis/gel coat blistering issues are very common on one of FPs main competitor but somehow they manage to keep it quite.
As far as our boats go, being made in two main pieces makes them lighter and stronger than other comparable build methods which join more pieces together to form the basic structure.
I don't know the exact fatigue life of GRP but if done properly it will be much higher than any metal or organic material like timber.
I have worked in aviation engineering for 30 years and I have never seen a GRP part fail because of fatigue. Plenty through impact damage or chaffing but not fatigue. I'm sure it will happen, but one of the biggest advantages of GRP is its flexibility and so fatigue resistance will always be high.
Rest assured our boats will be still going strong long after we have all departed this world.
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Old 16-10-2011, 18:05   #13
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Re: Catamaran Structural Resistance to Materials Fatigue

Yep. The thing must be built to the projected use: differently for one-time racing event, for charter work inshore, for repeated, extended offshore work.

This is one of the reasons why a serious cruising design will be 'overbuilt' - to limit flexing so that more cycles will be possible before failure. This is also the reason why many modern charter/marina queens can cross an ocean - while they are new and provided they will not be asked to do so too many times.

In a cat, like in most other boats, one can expect signs of trouble before any big drama. Inspect your boat - if there are no cracks or other signs of fatigue, she may yet take another passage.

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Old 16-10-2011, 18:15   #14
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Re: Catamaran Structural Resistance to Materials Fatigue

Well in the bar we say Fountaine Crackjot , nahh just kiding...
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Old 17-10-2011, 23:52   #15
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Re: Catamaran Structural Resistance to Materials Fatigue

Stopping flexing can often increase fatgue failure not reduce it.
Compare a fishing rod to a broom handle.
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